It is well-known that light pulses propagating along an optical fiber become broader and can eventually overlap one another. This phenomenon, which limits the information-carrying capacity of an optical fiber, is known as dispersion. Mathematically, dispersion is defined as the derivative of group delay with respect to wavelength, with group delay being the change in the optical signal propagation rate with change in wavelength. While dispersion is present in all optical fibers, the dispersion per unit length of single-mode optical fibers is quite small compared to multimode optical fibers. Accordingly, high-resolution techniques are required to characterize the dispersion in single-mode optical fibers. This characterization is especially important in the design and implementation of high-speed, single-mode optical fiber transmission systems.
A customary technique of determining dispersion in optical fibers is to measure the group delay spectrum of short, spectrally narrow pulses. This method is described in a publication by L. G. Cohen et al. in a publication entitled "Experimental Techniques for Evaluation of Fiber Transmission Loss and Dispersion", Proc. IEEE, 1980, 68 (10), pp. 1203-1209. A variety of other schemes are also known. For example, Crosignani et al. obained time-delay resolution of 10 psec. by cross-correlating modes in a multimode fiber using a frequency-modulated laser source. (See "Measurement of Very Short Optical Delays in Multimode Fibers", Applied Physics Letters, 1975, 27 (4), pp. 237-239.) Rashleigh and Ulrich, as described in a paper entitled "Polarization Mode Dispersion in Single-Mode Fibers", Optics Letters, 1978, 3 (2), pp. 60-62, measured the dispersion for different spatial polarizations in a single-mode fiber to less than 1 psec. resolution. A cross-correlation method, utilizing an interferometer to measure coherence time of a fiber relative to an air path, is described by N. Shibata et al. ("Spatial Technique for Measuring Modal Delay Differences in a Dual-Mode Optical Fiber", Applied Optics, 1980, 19 (9), pp. 1489-1492). This technique gave a resolution of 4 psec. in a dual-mode fiber. The resolution was extended to 0.1 psec. in further work. (See, for example, M. Tateda et al., "Interferometric Method for Chromatic Dispersion Measurement in a Single-Mode Optical Fiber," IEEE J. Quant. Elect., 1981, QE-17 (3), pp. 404-406, and W. D. Bomberger et al., "Interference Measurement of Dispersion of a Single-Mode Optical Fibre" Elect. Lett. 1981 (14), pp. 495-496.) Finally, a procedure developed by Shang ("Chromatic Dispersion Measurement by White-Light Interferometry on Meter-Length Single-Mode Optical Fiber", Elect. Lett., 1981, 17 (7), pp. 603-605) provides subpicosecond resolution and deduces the delay spectrum directly from a spectral scan of an interferometer output.
The problem with all of the above-described techniques is that they either require fairly complicated equipment, such as tunable dye lasers or elaborate holographic reconstruction or data analysis, or they require precise alignment by skilled personnel of the dispersion-determining apparatus as the source wavelength is changed. Consequently, none of the foregoing techniques is particularly suited for production line measurement of dispersion in optical fibers.